Easy enough to borrow or steal Tony's Jarvis tech and pattern it after his own brainwaves. One insertion into a custom built indestructible nanotech chassis later and voila!

Not that that's particularly clever of course. I did always like the idea that Tony based Jarvis personality on the Stark's family butler. But mostly I just like saying 'indestructible nanotech chassis'

The thing is I don't see Antman making Ultron, as even for a movie, a genius developing technology to be able to enlargen/shrink himself and then on the side make a robot who's powerful enough to fight the Avengers, too contrived for audiences.